1. Field of the Invention
The present invention is generally related to the field of coffee, and more particularly to the making of an improved liquid coffee. More specifically, the present invention is directed to a technique of stabilizing a liquid coffee product by inhibiting the development of acidity which occurs on storage. This is accomplished by treating the coffee extract with an alkali and thereafter neutralizing the solution to the desired pH. The alkali treatment hydrolyzes the lactones and esters which are present in solubilized coffee solids and converts them into stable salts. The lactones and esters are the acid precursors which are responsible for the increase in acidity of stored liquid coffee products.
2. Description of the Prior Art
The acidic nature of coffee and the role it plays in the ultimate quality of a coffee beverage is well documented. Although the acid fraction of a coffee extract generally constitutes from only about 10 to about 15% of the solids in the extract, the effect that acids have on the final coffee product can be said to be a determining factor in the consumer appeal of a coffee beverage. More particularly, too much acidity in a coffee can result in an undesirable sourness to the beverage, while too little acidity in the coffee causes a "flat" flavor profile. Finding and maintaining the right acidic balance is critical.
The task of adjusting the acidity of a coffee beverage is not made any easier by the fact that well over 25 different acids have been identified in roasted coffee. More particularly, about 8% of green coffee is comprised of numerous isomers of caffeoylquinic acids, also referred to as chlorogenic acids. Other principal acids found in green coffee include malic and citric acids. Roasted coffee produces additional acids, such as acetic, formic, glycolic, lactic and pyroglutamic acids.
Various factors are known which affect the acidity of a coffee extract. For example, different bean varieties result in varying acidity. The pH of a coffee brewed from Arabica varieties is generally found to be between 4.85 and 5.15. In contrast, coffee brewed with Robusta beans generally have a pH in the range of 5.25 to 5.40. Other factors which are said to influence the degree of acidity include the degree of roast, the type of roaster, the nature of the processing and the age of the green beans.
Liquid coffee products, although not widespread in the United States, represent a significant part of the Japanese and Korean coffee markets. The product is usually pre-sweetened and ready to drink. The liquid coffee product is generally prepared by mixing a diluted coffee extract with the desired additives, such as milk, sugar and flavorants. The product is then packaged in a suitable container, such as a can, which can be subjected to retort processing. The result is a liquid coffee product which can be distributed to the consumer. The product can be stored for generally up to six months at room temperature before consumption.
Unfortunately, a major problem exists with the marketing of liquid coffee. More particularly, coffee extract is an unstable system and both the shelf- and refrigerator-stored liquid coffee products currently available develop an increased acidity over a short period of time. Too low of a pH also results in the possible curdling of the milk or cream. Simply, the rise in acidity translates into a loss of quality to the product. That is, there is an increased sourness to the liquid coffee product. This quality loss is known as "staling" and although the cause of staling is attributed for the most part to the drop in pH and the increase in titratable acidity, no clear explanation or mechanism is known for its occurrence.
H. G. Maier, et al., Dtsch. Lebensmittel-Rdsch. 80(9): 265-268 (1984) have shown that the content of low molecular weight acids increase on storage at elevated temperatures and attributed the increase to the hydrolysis of esters and lactones produced on roasting.
One solution which has been used to prevent the problem of sourness development is the addition of sodium bicarbonate to elevate the initial pH of the product. However, the product pH of the sodium bicarbonate-treated liquid coffee product still falls on storage and has additional potential repercussions on product flavor.
Consequently, there is still a need for a process that prevents the accumulation of acidity in stored coffee extracts.